1. Field of the Invention
The present invention generally relates to a direct type apparatus for directly detecting tire inflation pressure, and more particularly to an apparatus in which a signal of tire inflation pressure directly detected by a pressure sensor is transmitted from a transmitter attached to a wheel of a tire to a receiver of a vehicle body to specify a cause for the pressure deflation of the tire.
2. Description of Related Art
As one of various types of apparatuses for detecting tire inflation pressure, a direct type has been known. In this type, a transmitter with a sensor such as a pressure sensor or the like is attached to each of wheels of a vehicle, and a receiver with an antenna is disposed in a vehicle body. Each sensor directly detects tire inflation pressure of a tire of the wheel. When the transmitter transmits a detection signal of the tire inflation pressure to the receiver through the antenna, the tire inflation pressure is analyzed in the receiver.
When the analyzed tire inflation pressure is lower than a predetermined threshold value, an alarm lamp is turned on for a driver. Therefore, the driver can know that the tire inflation pressure reaches a low level.
In this apparatus, only one predetermined threshold value is preset. Therefore, regardless of causes of the pressure deflation of a tire, the same alarm is given to the driver.
However, there are actually various causes for lowering tire inflation pressure of a wheel. For example, when a tire is suddenly punctured or blown out, immediate repairing of the tire is needed. When a tire is deteriorated, air of the tire leaks out through the deteriorated rubber of the tire, and tire inflation pressure is gradually lowered. In this case, no immediate repairing of the tire is needed, but the tire must be repaired in the near future. Further, a size of molecules of air is much smaller than that of rubber. Therefore, even tough the rubber is not deteriorated, air of a tire naturally leaks out through the rubber for a long time so as to lower tire inflation pressure very slowly. In this case, no repairing of the tire is needed, but recharging of the air in the tire is needed.
In this specification, the term of “deteriorated tire” indicates a tire in which grooves notched on rubber remain sufficiently deep but the rubber is deteriorated because of the long time passage of the tire from its production so as to cause air leaking from the tire in a degree exceeding the natural air leaking.
Another type of apparatus for detecting tire inflation pressure has been disclosed in Published Japanese Patent First Publication No. H07-323708 (1995). In this apparatus, tire inflation pressure is detected by using a Hall effect element with a magnet, and it is judged whether the pressure deflation of a tire is caused by a puncture of the tire or natural leaking of air. More particularly, a diaphragm is moved in response to a change of air pressure of an air chamber surrounded by a tire and a wheel of the tire, and the magnet is rotated with the movement of the diaphragm so as to change a magnet field. The Hall effect element detects the air pressure in response to the change of the magnet field and outputs a signal indicating tire inflation pressure. When the tire inflation pressure indicated by the signal is lowered to a predetermined threshold value, a change of the tire inflation pressure per unit time is calculated. When the change is larger than a predetermined change value, it is judged that the pressure deflation of a tire is not caused by the natural air leaking but is probably caused by a sudden puncture of the tire, and an alarm is outputted.
However, though the apparatus of the Publication can detect that the pressure deflation of a tire is not caused by natural leaking of air, the apparatus cannot correctly detect other causes. For example, when tire inflation pressure is lowered because of deterioration of a tire, the apparatus erroneously judges that the tire inflation pressure is lowered by a sudden puncture of the tire or natural leaking of air from the tire.
Further, in the apparatus of the Publication, even though tire inflation pressure is gradually lowered for a long time due to natural leaking of air, the apparatus cannot detect the natural leaking of air until the tire inflation pressure is lowered to the predetermined threshold value. In this case, until the tire inflation pressure is lowered to the predetermined threshold value, a driver cannot notice that he drives the vehicle at inadequate fuel economy.
Moreover, in the apparatus using the Hall effect element, tire inflation pressure is not directly detected, but detects air pressure of the air chamber as tire inflation pressure. Therefore, the tire inflation pressure cannot be detected at high precision. Further, when the air pressure of the air chamber is changed for a short time, the movement of the diaphragm does not correctly reflect a change of the tire inflation pressure. Accordingly, this apparatus cannot quickly determine whether the pressure deflation of a tire is caused by a sudden puncture of the tire or natural leaking of air from the tire.
Generally, tire inflation pressure of a wheel is gradually lowered because of natural air leaking. When tire inflation pressure of a wheel is lowered to a specific value, it is difficult for a driver to drive his vehicle safely or at adequate fuel economy. Therefore, it is necessary to inform a driver that tire inflation pressure of his vehicle will reach the specific value in the near future.